1. Field of the Invention
This invention relates to a fuel cell system integrated to utilize hydrogen produced by steam reforming of methanol.
2. The Prior Art
Fuel cells generate electricity through galvanic combustion of fuel process gas with oxidant process gas. Typically oxidant gas can be obtained from the fuel cell environment with little, if any, processing. The fuel process gas, on the other hand, is usually hydrogen and its generation requires processing of other fuels such as methanol. Direct oxidation of fuels such as methanol in fuel cells at practical current densities with acceptable catalyst loadings is not as economically attractive as conversion of methanol fuel to a hydrogen-rich mixture of gases via steam reforming and subsequent electrochemical conversion of the hydrogen-rich fuel stream to direct current in the fuel cell.
A very attractive fuel cell system currently undergoing commercial consideration is the reformed methanol fuel-phosphoric acid electrolyte-air system. Primary advantages of phosphoric acid electrolyte (85 wt. %) include ability to operate with fuel and ambient air containing CO.sub.2, ability to operate with a thin matrix electolyte (no liquid circulation) and chemical stability of the electrolyte over the operating temperature of the cell, e.g. 180.degree.-200.degree. C.
The fuel cell itself is only part of the overall system, and other components of the system, e.g., generation of hydrogen fuel, are likewise important in terms of overall system size and cost effectiveness.
In one method used by the art to produce hydrogen by steam reforming, a methanol and steam feedback is passed through catalyst filled tubes disposed within a reactor or reformer. Fuel and air are combusted outside of the tubes in the reformer to provide heat for the endothermic catalytic reaction taking place within the tubes at about 300.degree. C. In this process, the mixture of methanol and steam is converted to a gaseous stream consisting primarily of hydrogen (about 68%) and CO.sub.2 (about 21.7%), Co (about 1.5%) and H.sub.2 O (about 8.8%). In order to improve the thermal efficiency of such apparatus, efforts have been directed to improve the uniformity of heat distribution in the tubes within the reactor while minimizing the amount of energy used to produce each unit of hydrogen containing gas.
For the most efficient operation of the steam reforming reaction, large surface areas are required to transfer the heat from the combusted gases to the tubes. In reformers presently used for steam reforming, small diameter reaction tubes are clustered closely together in the furnace so that heat transfer from the combusting gases in the reactor into the catalyst packed tubes is optimized.
The use of a plurality of tubes to accomplish heat transfer contributes to the large size and high cost of the reformer. A second drawback to such reformers is that the heat for the steam reforming process is provided indirectly by means of heat transfer through tube walls. This inefficient heat transfer has a detrimental effect in fuel cell systems in which the reformer and the fuel cell are fully integrated, i.e. the combustion gases for the reforming reaction are derived from the fuel cell exhaust. Thus, at the inlet of the reformer, it is impossible, because of the highly endothermic nature of the reaction, to supply enough heat to the surface area of the reformer tubes so there tends to be a large decrease in reactant temperature in the area adjacent the inlet. A large portion of each reactor tube, as a result operates at an undesirably low reaction temperature. The resultant effect of the fuel cell system is that, in order to effect complete methanol conversion, the reformer must necessarily be of a large size and concomittant high cost.
In copending patent application Ser. No. 743,204 filed of even date herewith, the production of hydrogen by the steam reforming of methanol is accomplished in a reformer of substantially reduced size by superheating a gaseous mixture of water and methanol at a temperature of about 800.degree. to about 1100.degree. F. and then feeding the superheated gaseous mixture to a reformer in contact with the catalyst bed contained therein, whereby at least a major portion of the heat for the endothermic steam reforming reaction is provided directly by the sensible heat in the superheated steam/methanol stream.
As a result of direct heating of the reformer feed gases augmenting indirect heat transfer through the wall of the reactor, a shell-and-multiple tube reactor arrangement or other means of increasing the heat transfer surface is not always required and the complexity and overall volume of the reformer can be substantially reduced. This invention is based upon the realization that an efficient practical integrated reformer-fuel cell system and process can be achieved by using a superheater, an essentially adiabatic methanol reformer and a fuel cell wherein the various exhaust streams from the components are utilized with other components of the system. In this system, the water to methanol ratio and temperature of the stream leaving the superheater are of such values that substantially all (at least about 75%, preferably 90%) of the heat required for the endothermic reforming reaction is contained within the reaction stream itself and, at most, only a small portion of the heat required for reforming is supplied through the wall of the reforming reactor. In addition to the reformer size reduction achieved by the use of the process disclosed in copending U.S. Ser. No. 743,204, a further reduction in reactor size is achieved by use of an essentially adiabatic reactor. This invention is also based on the realization that it is highly advantageous to integrate the steam reforming process disclosed in copending U.S. Ser. No. 743,204 with a fuel cell to form a fuel cell system whereby a continuous supply of hydrogen could be provided to the fuel cell from an essentially adiabatic steam reformer, the gases exhausted from the anode of the fuel cell providing thermal energy via combustion for superheating the water/methanol mixture.
It is therefore a primary object of the present invention to efficiently integrate a fuel cell with the steam reforming process to provide a thermochemical process for producing electrical energy in which the heat required for the endothermic reforming reaction is contained substantially completely within the stream fed to the reformer which produces hydrogen for a fuel cell so that a compact and efficient system may be obtained.
The above object of the invention is achieved in accordance with the fuel cell system of the present invention comprised of a heat exchanger, a burner, an adiabatic steam reformer and a fuel cell wherein a superheated mixture of water and methanol is first converted by an essentially adiabatic endothermic catalytic reforming reaction to hydrogen. The hydrogen, generated in the reformer, is directed to the fuel electrode of the fuel cell, and air is directed to the oxygen electrode to effect an electrochemical reaction to produce electricity and gaseous reaction products. A portion of the exhaust gases from the fuel electrode is combustible as it contains unreacted hydrogen. Furthermore, it is desirable to withdraw this portion of gas from the fuel cell to maintain a hydrogen-rich stream in the fuel cell thus optimizing fuel cell operation in accordance with the present state of the fuel cell art. The combustible gas exhausted from the fuel electrode is burned in the burner, the exhaust of which is fed to the heat exchanger to supply heat for superheating the water/methanol mixture fed to the reformer. Even though large amounts of water are used in the system and thus more heat is required to vaporize and preheat these methanol water mixtures containing large amounts of water, the heat generated can be effectively recovered and used in the system and process of the present invention. Further, parasitic power requirements are decreased, and the low concentration of carbon monoxide in the reformate should lead to improved fuel cell efficiency and extended fuel cell life, so the net methanol demand remains essentially constant or may decrease slightly (around 20%). Thus, the net energy production is at least substantially equivalent to that obtained using mixtures containing lesser amounts of water.